All your hard work spent mining has made you exceptionally tough, increasing your Stamina by X.

Toughness is a passive trait exclusive to miners. It increases your Stamina, with patch 3.0.8, scaling with mining skill level.

This is roughly the same bonus to stamina as that given by the crafting professions, making mining very attractive to tanks. The total stamina can of course be increased additionally with Talents, such as Survivalist and Vitality.